Is there anything you can do besides run CCleaner, do a virus scan (I use AVG free), update drivers, and make sure you have the latest important Windows Updates? Because I'm trying to figure out if I'm lagging because my hardware isn't good enough or if it's another problem such as something CCleaner would fix.
You may be hitting the ceiling when it comes to RAM. I had 4 GB for quite a while and the system ran fine. However, once I upgraded to 8 GB, games became much smoother and easier to start. Basically, you may not see your PC hitting the ceiling, but usually during app launches, a lot of RAM is used to "set up" the app.
You could overclock you CPU too. Pretty simple with a C2Q (I would know). You would need an after market cooler.
Your GPU is a little old too, but should not have any issue playing those games let alone Youtube.
I looked into this for myself. I was going to buy one at Thanksgiving. But after some reading, I didn't think it was worth it considering my PC is SATA II not III. It would improve my PC a lot but I didn't think it was necessary until I get SATA III.